Btw. I just tried that and it works, you just may want to use alt + o to start the game as when you click "start" there is very little time to go to observers box. Of course do that on multiplayer (even if offline)

If you have v1.16.1, ChaosLauncher has a plugin that allow you to observe on maps that is not even designated for observers, and with some nice features that is not even exist on the poor SC:R, you can check it here.

"All who believe in Allah and the last day, either say good or be silent." Muhammad